It has been an historic few days at the Supreme Court, as the justices have weighed the legality of a landmark health care law. The court will hold its third and final day of arguments today. President Obama's supporters see the law as a signature achievement in his first term but now it hangs in the balance.

As the drama has played out this week, there are people with offices right across the street from the court who've been watching very closely. They are members of Congress.

NPR's Ari Shapiro has this report.

ARI SHAPIRO, BYLINE: You could call it the Washington equivalent of a major Hollywood premiere. But instead of red carpets and paparazzi, there were bullhorns and marching protesters.

(SOUNDBITE OF PROTESTS)

SHAPIRO: The coveted seats inside the chamber did not go to movie stars in glittering ball gowns. They went to buttoned-up men and women dressed to blend in with the crowd.

SENATOR MIKE LEE: I've been watching Supreme Court arguments since I was 10 years old. It something of a hobby of mine from the time I was a kid.

SHAPIRO: Senator Mike Lee of Utah is a Republican who identifies with the Tea Party movement. He clerked for Judge Samuel Alito before Alito became a Supreme Court justice.

LEE: I have never seen an oral argument that lasted two hours in one sitting. I've certainly never seen a case with a total of about six hours, which this case will end up consuming. So that by itself was very significant.

SHAPIRO: Lee believes the health law goes beyond Congress's powers. And he was pleased to see many of the justices hint at similar views during oral arguments - for example, this was Justice Anthony Kennedy in the courtroom.

(SOUNDBITE OF SUPREME COURT HEARING)

SHAPIRO: Lines like that make Senator Lee optimistic.

LEE: Justice Kennedy will be the swing vote, he'll be the fifth vote for a 5-4 decision one way or another, I'm quite sure. And I'm reasonably confident, based on his questions, his reactions to the questions, and based on his prior writings, both for the court and in dissenting opinions and concurring opinions, he's going to side with the conservatives. He's going to say this is beyond Congress's Commerce Clause power.

SHAPIRO: The government needs to pick off at least one of the court's conservatives in order for the health care mandate to survive. From the arguments, none of them seems to be leaning that way.

Democratic Congressman Sandy Levin of Michigan was unfazed.

REPRESENTATIVE SANDY LEVIN: I think everybody did well enough. I don't think the outcome will be determined by the argument - by the manner of the argument presented.

SHAPIRO: Some legal experts agree that oral arguments don't influence justices much. But arguments can certainly provide a window into the court's thinking. And in yesterday's back and forth, the health care law's defenders seemed to have a steeper climb than the challengers.

Nevertheless, Senator Tom Harkin, a Democrat, came away as confident as ever that the Constitution is on the law's side. He says the government has decades of case law behind it, and he says the health care system uniquely demands congressional involvement.

SENATOR TOM HARKIN: It kept being brought up. You know, we have 30 to 40 million Americans who have no health care insurance, but they will use health care.

SHAPIRO: Like a red carpet premiere, yesterday's event attracted Washington figures of every stripe. Congresswoman Michele Bachmann showed up at a Tea Party rally on the courthouse steps and weighed in on the impact of this case on the Republican presidential race.

REPRESENTATIVE MICHELE BACHMANN: Now every candidate that remains in the race, whether it's Romney, Santorum, Gingrich or Paul, they all say that they will fully repeal Obamacare. Now it's up to us to hold these candidates' feet to the fire.

SHAPIRO: She predicted that if the Supreme Court upholds the health care law, the phrase Repeal Obamacare will become Republicans' number one battle cry. If the Supreme Court strikes down the law in the middle of the presidential race - well, then Republicans may just need to find a new refrain.